System for controlling or adjusting an electromechanical brake

ABSTRACT

There is the problem of adjusting a clearance in electromechanically or electromotively operated brakes because automatic resetting means, which are provided for hydraulic brakes, cannot be used in brakes of the above type. 
     Initially, it is necessary to determine a neutral position of the motor which is defined by the friction elements just abutting the brake disc in this position. 
     This situation is determined by a detection device which exhibits the following operation in one preferred embodiment. The motor is driven from an unknown position by a constant motor torque so that the friction linings are moved towards the brake disc. The motor will speed up because the resistances remain constant in the starting stroke. An abrupt deceleration of the motor will be caused when the friction linings are applied to the brake disc. The zero passage of the angular acceleration can be sensed and interpreted as an application of the friction linings on the brake disc. Subsequently, a so-called contact signal is generated, with the result that the motor is restored in a position-controlled manner by a defined angle which corresponds to the clearance being adjusted.

This application is filed under 35 U.S.C. § 371 from InternationalApplication PCT/EP96/03978, with a filing date of Sep. 11, 1996, furtherclaiming priority on German Application No. 195 36 695.6, filed Sep. 30,1995.

BACKGROUND OF THE INVENTION

The present invention relates to a system for controlling or adjustingan electromechanical brake.

An electromotive brake with these features is described in U.S. Pat. No.4,995,483. The system disclosed is used to decelerate aircraft, however,its principal structure is applicable on road vehicles as well. Thebrake includes a set of brake discs and an associated set of frictiondiscs which are urged against each other by way of a clamping device.The clamping device is actuated electromotively by way of a spindle,which is driven by a roller thread drive and pressed against the outwardfriction disc. A force-measuring element is interposed between the firstfriction disc and the spindle head. A clearance is provided between thebrake discs and the friction discs.

The general point is that to produce a brake force, first the clearancemust be overcome. Only after the friction discs or friction elementsabut on the brake discs is it possible to transmit a clamping forcewhich causes deceleration of the wheel connected to the brake discs. Toapply the friction discs on the brake discs, i.e., to overcome theclearance, only low forces are transmitted which should not exceed adefined limit value.

As soon as a limit value is exceeded, the system disclosed in the aboveU.S. patent interprets this fact as application of the friction discs onthe brake discs.

The associated angular position of the driving electric motor is definedas the zero position.

To produce a clamping force, the motor can be readjusted by definedamounts of angle, and the interrelation between the readjustment of themotor based on the zero position and the clamping force exerted is takeninto account.

When the brake is released, the spindle is initially restored until thezero position is reached. Subsequently, the spindle is reset by afurther amount which corresponds to the clearance. This type ofclearance adjustment is very inaccurate, and, in addition, it may onlybe effected during brake application. It is not possible to adjust theclearance independently of a braking operation.

The provision of a force sensor is absolutely necessary in the brakedisclosed in the above-mentioned U.S. patent in order to be able todetermine the neutral or zero position. The problem is that the signalof such force sensors is subjected to a drift so that it is onlypossible to determine the actual force exerted by using major electronicmeans. Therefore, the objective of development is to obviate the needfor a sensor of this type and to derive the necessary data for brakeapplication from the signals of other sensors, for example, a sensorwhich senses the wheel rotational speed.

However, this eliminates the possibility of performing the method ofdetermination of the neutral position mentioned in the above U.S.patent.

Therefore, an object of the present invention is to provide acontrolling or adjusting system which permits adjustment of a clearanceeven without the use of a force sensor.

Another object of the present invention is to provide a controlling oradjusting system to identify and readjust the clearance which operatesindependently of brake application and, in addition, permits readjustingthe clearance even during travel of the automotive vehicle.

SUMMARY OF THE INVENTION

To be able to adjust the clearance and to actuate the brake in a definedfashion, it is necessary to detect in which angular position, i.e.,neutral position, of the motor the friction elements are applied to thefriction surface in order to define the neutral position.

The application of the friction element against the friction surface isdetermined during a so-called detection movement by means of a detectiondevice. When the detection device produces a contact signal, theassociated angular position of the driving electric motor can be definedas zero position or neutral position φ₀. Subsequently, the frictionlining is controlledly reset to a position where a previously definedclearance Δφ is maintained.

‘Position control’ means that the electric motor is actuated so that itrotates at an angular velocity which is predetermined by a controller.The controller calculates a nominal angular velocity value from thedifference between the desired nominal motor angle value and the actualmotor angle value prevailing. This way, the nominal clearance can beadjusted quickly and sensitively.

The detection device which determines whether the friction linings bearagainst the brake disc can be achieved in two ways.

One possibility involves actuating the electric motor with anapproximately constant motor torque. To this end, the motor is driven bya constant nominal motor current value which produces an approximatelyconstant motor torque in motors which are usually employed for actuatingdevices of this type.

It can be expected in this mode that the motor rotates at an increasingrate of angular velocity as long as the friction element is still in theclearance, i.e., is spaced from the friction disc. The application ofthe friction elements on the friction surface of the brake disc may nowbe determined as follows. The angular velocity and the angularacceleration of the motor shaft is observed while the clearance isovercome. The signals of a resolver are taken into account for thispurpose. The signals of the resolver are also used to perform electroniccommutation of the motor.

The motor torque is so adjusted that it only slightly exceeds thefriction torque of the motor including the coupled mechanics. Theremaining effective torque causes a low acceleration of the motor shaftand an accelerated approaching movement of the friction linings orelements in the direction of the brake disc. The result is that, after apreviously unknown travel has been covered, the friction linings areapplied with low force to the friction surface of the brake discs. Theresulting load torque of the motor initially causes a reducing motoracceleration and, subsequently, deceleration (negative acceleration) ofthe motor angular velocity until standstill. Because the motor torquepredetermined for this detection movement generates only a small motoracceleration torque, the application of the friction linings on thefriction disc effects already an almost immediately occurring signreversal of the motor acceleration.

The zero passage of the motor acceleration is used as a contact signalfor applying the friction linings against the friction surface. Themotor angle at the point of time of zero passage thus achieves theneutral or zero position of the motor.

Because the effective motor torque adopts a very small value only, theapplication of the friction linings on the friction surface will notproduce any considerable deceleration of the wheel. Thus, the basic ideais to move the friction lining with low force against the frictionsurface, so that major forces which would cause deceleration of thevehicle are not transmitted upon application of the friction element onthe friction surface.

A second possibility of realizing the detection device involves theprovision of contact pins in the friction linings which, when applied tothe brake disc, close a current circuit and thereby produce a contactsignal in the detection device. The detection movement isposition-controlled in this case, i.e., it is performed at a controlledangular velocity.

The contact pin arranged in the friction lining extends through thefriction lining towards the brake disc and, therefore, is exposed to thesame wear as the friction lining itself.

It is an advantage in both mentioned methods of determining the neutralposition that there is no need for a force sensor.

The special advantage of the method of determining the neutral position,where the variation in the angular velocity of the motor is monitored,is that the signals of a rotation sensor can be used which is alreadyprovided and required for the electronic commutation of the motor.

When the detection device issues a contact signal, the friction liningin both methods is restored in a position-controlled fashion to aposition where a previously defined clearance is maintained. In thereturn stroke, i.e., when the actual clearance is adjusted, the motor isso regulated that it rotates at an angular velocity which ispredetermined by a controller. The controller calculates a nominalangular velocity value from the difference between the nominal motorangle desired and the actual motor angle. This way, the nominalclearance may be adjusted quickly and sensitively.

The idea of the present invention will be explained in detailhereinbelow by way of several Figures.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings,

FIG. 1 is the typical structure of a wheel brake operated by an electricmotor.

FIG. 2 is a block diagram relating to the actuation of the motor.

FIGS. 3, 4 are arrangements to identify whether the friction liningbears against the brake disc.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an electromechanically operable floating-caliper discbrake. Of course, the actuation of the brake described hereinbelow mayalso be transferred to all other types of disc brakes and to drumbrakes.

The floating-caliper disc brake includes a brake caliper 1 which isslidably mounted with respect to the brake disc 3, and an electric motor2 having its housing 8 attached to the brake caliper 1. One frictionlining 4, 5 is arranged on either side of the brake disc. The firstfriction lining 4 is supported on a spindle 14 which is driven by theelectric motor 2, with the result that the lining can be pressed againstthe brake disc 3 by the electric motor 2. The second brake lining 5 isattached to the brake caliper 1 and pressed against the other side ofthe brake disc by reaction forces which are transmitted to the caliper 1when the first lining 4 is applied to brake disc 3.

The electric motor 2 is a permanently energized, electronicallycommutatable motor having a stator 9 rigidly mounted in the housing 8.The stator 9 is configured as a coil system in a known fashion. Rotor 10includes a hollow shaft 15 which is rotatable in the brake caliper.Several permanent magnetic segments 29 are arranged on the outside ofthe hollow shaft and rigidly connected with the hollow shaft 15. Themagnetic field produced by the stator 9 interacts with the permanentmagnetic field of the magnetic segments 29 and causes rotation of thehollow shaft 15. The rotation is transferred into an axial movement ofthe spindle 14 by a roller screw drive. To this end, the inside of thehollow shaft 15 and the outside of the spindle 14 each include a threadengaging into the thread of paraxial threaded rollers 12, 13.

The spindle 14 is configured as a hollow cylinder with an inward step20. Bearing against step 20 is a spherical cup 22 on which a push rod 23abuts. Rod 23, in turn, is supported with its other end on anotherspherical cup 22A which is anti-torsionally connected to a liningcarrier 24 of the first friction lining 4.

Further, the motor 2 includes a so-called resolver 30. Resolver 30 hastwo rings 31, 32 which are coaxial to each other and separated by an airslot. The radially inward ring 31 is connected to the hollow shaft 15 bya retaining element 33. The radially outward ring 32 is connected to thehousing 8. The signals of the resolver are used, on the one hand, tocommutate the motor, i.e., to perform the energization of the stator 9so that a traveling magnetic field is generated and, on the other hand,to determine the relative angular position of the rotor. Because theangular position of the rotor is directly linked to the axial positionof the spindle, the signal of the resolver is also an indicator of theposition of the spindle 14 in the brake caliper 1.

The brake described hereinabove is actuated by energization of theelectric motor 2.

The brake must perform the following functions:

1. Apply the brakes

This means that the friction linings 4, 5 are pressed against the brakedisc with a defined force, the clamping force, so that the frictionforces generated produce a brake torque which causes deceleration of thevehicle wheel connected to the brake disc 3. When the brake is applied,a nominal angular velocity is adjusted which is produced from thedifference between a nominal deceleration signal and an actualdeceleration signal so that the motor is acted upon by a current of astrength at which the desired angular velocity is achieved.

2. Release the brakes

This means that the clamping force is decreased and, thus, adapted tothe vehicle deceleration desired by the driver.

3. Adjust the clearance

This means that the electric motor, after braking, must be actuated sothat the brake linings 3, 4 maintain a distance from the brake disc.This distance is termed as ‘clearance’. The purpose of maintaining aclearance of this type is to prevent the friction linings from rubbingagainst the brake disc when a braking effect is not intended.

As indicated in FIG. 1, the second friction lining 5 includes a contactpin which extends through the lining to the disc. A similar contact pinis provided for the first brake lining 4 which is not shown in FIG. 1.When the brake linings bear against the disc, as shown in FIG. 1, aconductive contact is made between the tips of the contact pins and thebrake disc. A current circuit may so be closed, as will be explainedhereinbelow.

The resolver 30 permits a relative angle measurement for the angularposition of the rotor sleeve 15 and, thus, the position of the spindle14. The design of the mechanics described in FIG. 1 does not provide fora direct technical measurement of the clearance. However, therelationship between the axial position of the spindle and the brakeapplication force varies due to lining wear and thermal effects. Anotherproblem is that the current clearance situation is absolutely unknown inthe beginning when the actuation for the electromechanically operablebrake is switched on. This necessitates determining the adjustment ofthe clearance immediately after the actuation is switched on. When theclearance is known, braking may be performed far more sensitivelybecause the transition to proper braking can be effected at the correctangular position. The knowledge of the exact size of the clearance isnecessary especially when the brake force to be exerted must becontrolled rather than regulated.

The actuation of the electric motor and the associated brake is shown inthe FIG. 2 embodiment to determine the current clearance situation andto adjust the clearance. The energization of the electric motor 2 iseffected by a so-called electronic servo booster 40. The input 41 of theservo booster is connected to a monitoring module 42 by two paths.Operation of the servo booster may be in two modes. The nominal angularvelocity of the motor is applied as an input variable at input 41. Aninternal final stage supplies the motor current for the electric motorat output 43. The motor current may now be adjusted so that thepredetermined angular velocity or the predetermined angular velocityvariation is maintained at the input 41. There is an internal feedbackof the resolver signal 30 in the servo booster. The servo booster thusperforms in the ‘motor rotational speed control’ mode of operation. Asignal S=1 is applied to the control input 44.

The servo booster can also be operated in the ‘motor torque control’mode of operation. In this mode, the feedback in the servo booster isinternally deactivated so that the strength of the motor current whichprevails at output 43 depends only on the quantity at the input 41. Thismode is achieved when the value S=0 is applied to the control input 44.In this mode of operation, the nominal value for the motor torque isapplied as input quantity to the input 41.

The monitoring module 42 furnishes a nominal motor torque signalM_(M,soll) 51 at a first output and a nominal angle signal φ_(soll) 52at a second output.

The control signal S is set to 0 or 1 at a third output 53.

The contact signal is applied at input 55.

The controlling and adjusting system for determining the currentclearance situation and adjusting the clearance is activated by acontrol variable at the input 54 which is set by a superior functionunit (not shown) if necessary, for example, if the objective is to senseand re-adjust the clearance.

To be able to adjust a new, adapted clearance, a new neutral positionmust be determined first of all. It is necessary to generate a contactsignal so that the angular position of the motor at the time ofgeneration of the signal can be defined as the new neutral position.

A new neutral position can be determined in various ways.

One possibility is described in the following. Initially, the signal S=0is set at the control output 53. Thus, the servo booster operates in the‘motor torque control’ mode of operation. With S=0, a switch 60 in thepath between the output 52 of the monitoring module 42 and the servobooster 40 is interrupted. Only the signal at the output 51 is sent byway of the open switch 61 to the input 41 of the servo booster. Thelatter drives the electric motor 2 such that the friction lining ismoved in the direction of the brake disc.

Starting from a previously defined initial value, the nominal torque isincreased until a movement is reliably identified by way of the motorspeed signal or a defined top limit for the nominal torque is reached.

When the top limit is reached, the brake is obviously in an appliedcondition. S=1 is set in this case, and the electric motor is restoredby an angular variation in a position-controlled manner. A new attemptto determine the neutral position is made after the movement iscompleted.

When a movement of the motor occurs, the clearance detection isactivated after a predetermined minimum angle variation. Taking intoaccount a minimum angle variation prevents an only short-time movementdue to static friction or sliding friction effects or due to alreadyprevailing but low load torques. Such a short movement does not permitreliable detection. The angular acceleration is monitored afteractivation.

Because the servo booster 40 operates in the ‘torque control’ mode ofoperation upon application of the brake linings, i.e., is driven at aconstant motor torque, the angular velocity of the motor will risecontinuously due to the predefined acceleration torque. As soon as thefriction element abuts the brake disc, the load of the motor will riserapidly so that the angular velocity of the motor will be retardedrapidly until standstill. The angular velocity passes a zero value. Thezero value can be recorded in the detection device 56 and evaluated as acontact signal, i.e., as an indicator of abutment of the brake liningson the brake disc. The contact signal is applied to the input 55 of themodule 42.

Another possibility includes the arrangement of a contact detector shownin FIG. 3. Both brake linings 3 and 4 have each one contact pin 70, 71which extend through the respective lining and end with the frictionsurface of the linings. When the friction linings are applied to thedisc 3, the contact pins will also move into contact with the brake discand close a circuit.

In the FIG. 3 embodiment, the contact pin 70 is connected to a pole of avoltage source 72, and the contact pin 71 is connected to the other poleof a voltage source 72. The circuit includes a light-emitting diode 73the light of which strikes a light-sensitive element 74.

When the light-sensitive element is rendered conductive, a signalprevails at an output 75 (after an inverting action in an inverter 76)and the signal is evaluated as contact signal K and applied to the input55.

The galvanic separation of the circuits is necessary because the brakedisc is connected to the ground potential of the vehicle electricalsystem.

Another wiring possibility is shown in the embodiment of FIG. 4. Eachcontact pin 70, 71 has its own circuit in the vehicle electrical system.The connection to the ground potential of the vehicle electrical systemis made when the brake linings are applied to the brake disc so thatboth contact pins are connected to ground. Voltage is applied to thecorresponding signals in the inverters 80 and 81, which signals arecombined to a contact signal in an AND-element 82.

Consequently, a contact signal is in both cases present only when bothfriction elements bear against the brake disc.

Because the clearance adjustment strategy described hereinabove providesa defined testing cycle with defined test movements, an instantaneousreadjustment due to wear is not possible after release of the brake onthe basis of this method. Where the objective is to apply theabove-mentioned method for the wear readjustment, readjustment must bemade separately. It should be taken into consideration that a separatereadjustment is not absolutely necessary after each braking operation.The moment when wear readjustment is necessary can be defined by way ofrelevant signals provided by measurements and their evaluation over along period.

What is claimed is:
 1. A control system for an electromechanical brakewhich includes a first friction surface operably connected to an elementto be braked, a second friction surface provided on a friction elementwhich is movable against the first friction surface by an electric motorhaving a rotor, wherein the first and the second friction surfacesmaintain a predetermined clearance in between each other, a sensor whichdirectly or indirectly senses the angle of rotation of the rotor of theelectric motor, a contact-detection means which is able to determinewhether the first friction surface bears against the second frictionsurface and thereupon generates a contact signal, wherein a variation ofthe angular velocity of the rotor upon application of the first frictionsurface on the second friction surface is evaluated to determine therotor angle where the first friction surface touches the second frictionsurface in a neutral position.
 2. A system as claimed in claim 1,wherein for determining the neutral position the motor is driven at aconstant torque so that the second friction surface is moved towards thefirst friction surface, and wherein a zero passage of the angularacceleration is used as a basis for the generation of a contact signal.3. A system as claimed in claim 1, wherein the motor is adapted to beoperated either by a defined rotational speed or with a defined torque.4. A system as claimed in claim 3, wherein upon response of thecontact-detection means the motor is restored to the predeterminedclearance by control of the angular velocity.
 5. A system as claimed inclaim 3, wherein the electric motor is driven by a servo booster whichcan be switched by a switch element either to a ‘torque control’ mode ofoperation or a ‘rotational speed control’ mode of operation.
 6. A systemas claimed in claim 5, including a module which issues correspondingswitch signals to the servo booster.
 7. A control system for anelectromechanical brake which includes a first friction surface operablyconnected to an element to be braked, a second friction surface providedon a friction element which is movable against the first frictionsurface by an electric motor having a rotor, wherein the first and thesecond friction surfaces maintain a predetermined clearance in betweeneach other, a sensor which directly or indirectly senses the angle ofrotation of the rotor of the electric motor, a contact-detection meanswhich is able to determine whether the first friction surface bearsagainst the second friction surface and thereupon generates a contactsignal, wherein a variation of the angular velocity of the rotor uponapplication of the first friction surface on the second friction surfaceis evaluated to determine the rotor angle where the first frictionsurface touches the second friction surface in a neutral position, andwherein for determining the neutral position the motor is driven at aconstant torque so that the second friction surface is moved towards thefirst friction surface, and wherein a zero passage of the angularacceleration is used as a basis for the generation of a contact signal.